1. Field of the Invention
The present invention relates to a self-diagnosis apparatus for an exhaust recirculating system of an internal combustion engine such as an automotive engine and more particularly, to a self-diagnosis apparatus capable of exactly diagnosing the state of the exhaust gas recirculating system through an appropriate discrimination of a change in the intake pressure which is caused by, for example, a change in the engine load or a shifting operation of a transmission.
2. Description of the Related Arts
FIG. 6 schematically shows the construction of a known self-diagnosis apparatus of the type described above, which is disclosed in, for example, Japanese Patent Laid-Open No. 2-9937. Referring to this Figure, an internal combustion engine 1 has a combustion chamber 3, and an intake passage 2 and an exhaust passage 4 which communicate with the combustion chamber 3. The intake passage 2 comprises a throttle body 5 and an intake pipe 6. A surge tank 7 for accommodating pulsation of an intake pressure in the intake passage is disposed between the throttle body 5 and the intake pipe 6. The intake passage 2 and the exhaust passage 4 are connected to each other through an exhaust gas recirculating passage 8. An exhaust gas recirculation control valve (EGR valve) 9, which operates based on a pressure differential between the atmospheric pressure and the intake pressure, is disposed in the exhaust gas recirculating passage 8. The exhaust gas recirculation control valve 9 has a function to open and close the exhaust gas recirculating passage 8 in accordance with the state of operation of the engine 1 so as to introduce part of the exhaust gases from the exhaust passage 4 into the intake passage 2. Thus, the exhaust gas recirculation control valve 9 forms a critical portion of the exhaust gas recirculating system. The intake system of the engine 1 has a throttle valve 10 which is disposed in the throttle body 5 at a location upstream of the surge tank 7. An exhaust gas recirculation control port 11 opens to the intake passage 2 at a position upstream of the surge tank 7. The control port 11 communicates with a vacuum chamber defined in the exhaust gas recirculation control valve 9 through a vacuum passage 12. The vacuum passage 12 is provided with a vacuum switching valve which controls the operation of the exhaust gas recirculation control valve 9 in accordance with the level of a load on the engine 1. A pressure sensor 14 is selectively connected to one of the ambient air and a portion of the internal space in the intake passage 2 downstream of the surge tank 7, through a change-over valve 15, which switches a pressure detection region, for sensing an internal pressure in the intake passage 2. A microcomputer 16 receives signals from various sensors and performs various kinds of control such as the control of a fuel injection rate. To this end, the microcomputer 16 has a central processing unit 17, a memory 18, and input/output interfaces 19, 20. A throttle opening sensor 21 is disposed upstream of the surge tank 7 for sensing an opening of the throttle valve 10. A water-temperature sensor 22 measures the temperature of cooling water circulated through the engine 1. The engine 1 is provided with a distributer 23 which controls the timing of ignition. The distributer 23 has a crank angle sensor 24 which detects the angle of rotation of the crankshaft of the engine 1. The engine has a plurality of cylinders-each defining therein the aforesaid combustion chamber 3. Each cylinder is provided with a fuel injector 25 which injects fuel into a corresponding combustion chamber 3.
The operation of the above-mentioned known self-diagnosis apparatus for the exhaust gas recirculating system shown in FIG. 6 will be described with reference to FIG. 7, which shows a flow chart illustrative of the operation of the self-diagnosis apparatus. Step S1 determines whether or not the engine 1 is being decelerated. More specifically, the microcomputer 16 determines that the engine 1 is being decelerated, when the throttle valve 10 is in the fully closed position while the engine speed is higher than a prescribed speed which has been set in a light-load region, on condition that the temperature of the cooling water is higher than a predetermined temperature which has been set in a low-temperature region. The microcomputer 16 then operates to temporarily stop the supply of fuel from the fuel injector 25. When the microcomputer 16 has determined in Step S1 that the engine 1 is not being decelerated, the process proceeds to a main routine, whereas, if not, the process proceeds to Step S2. In Step S2, electrical power is supplied to the input terminals of the vacuum switching valve 13 to close it. The process then proceeds to Step S3 in which a determination is conducted as to whether the temperature of the cooling water exceeds a set temperature, e.g., 80.degree. C., based on the signal from the water temperature sensor 22. If the set temperature is not exceeded, the process proceeds to the main routine, whereas, if the set temperature is exceeded, the process goes to Step S4. In Step S4, a determination is conducted as to whether the engine speed NE is within a predetermined range between NE1 and NE2, based on the engine speed signal coming from the crank angle sensor 24. If the engine speed NE does not fall within the-above-mentioned predetermined range, the process skips to the main routine, whereas, when the engine speed NE is found to fall within the above-mentioned range, Step S5 is executed.
In Step S5, the intake pressure PM immediately before the vacuum switching valve 13 is switched from close to open state is set in the memory 18 at an address PM1. The process then proceeds to Step S6. In Step S6, the now closed vacuum switching valve 13 is opened so as to enable introduction of vacuum into the vacuum chamber in the exhaust gas recirculation control valve 9. The process then proceeds to Step S7 in which the intake pressure PM immediately after the opening of the vacuum switching valve 13 is stored in the memory 18 at an address PM2. Step S8 is then executed to determine whether a pressure differential between the intake pressure PM1 measured immediately before the vacuum switching valve 13 is opened and the intake pressure PM2 as measured immediately after the opening of the valve 13 is greater than a set value P1 which is, for example, 60 mmHg. If the pressure differential is smaller than the set value P1, the process proceeds to Step S9 which determines that a failure has taken place in the exhaust gas recirculating system including the exhaust gas recirculation control valve 9, and sets the fact of occurrence of the failure in a predetermined address of the memory 18. The process then proceeds to the main routine. When the pressure differential is greater than a set value P1, the microcomputer 16 determines that there is no failure in the exhaust gas recirculating system, and the process is advanced to the main routine.
In the known self-diagnosis apparatus for the exhaust gas recirculating system as described, an exhaust gas recirculation control valve, which is disposed in the exhaust gas recirculation passage inter connecting the intake and exhaust passages, is caused to temporarily open and close during deceleration of the engine, and occurrence of any failure in the exhaust gas recirculating system is determined based on whether or not the amount of change in the intake pressure caused by a change in the operating state of the exhaust gas recirculation control valve falls within a predetermined range. This known self-diagnosis apparatus, however, suffers from the following problem when used with an engine having such a speed control function that controls the engine speed through varying the flow rate of intake air against a change in the level of loads such as an air conditioner, a power steering gear and the like. In such a case, the variation in the intake flow rate also causes a change in the intake pressure, so that the self-diagnosis apparatus cannot discriminate the change in the intake pressure caused by the operation of the exhaust gas recirculation control valve from changes in the intake pressure resulting from the engine speed control. Thus, the self-diagnosis apparatus may erroneously take any change in the intake pressure caused by the engine speed control as being a sign of a failure which is taking place in the exhaust gas recirculating system detected through the operation of the exhaust gas recirculation control valve. This results in failing to exactly diagnosing the state of the exhaust gas recirculating system.
The intake pressure is also apt to change due to a change in the engine speed caused by, for example, a shifting operation of a transmission associated with the engine. Such a change in the intake pressure may also cause erroneous diagnosis of the exhaust gas recirculating system.